A USB device and a firmware updating method for the USB device are provided. The firmware updating method includes the following steps. Firstly, a communication protocol is provided. Then, the communication protocol is installed in the at least one microcontroller unit of the USB device. Then, an application program is produced according to the communication protocol. The application program is installed in an electronic computer. The application program contains at least one update firmware information. Then, the at least one update firmware information is transmitted from the electronic computer to the at least one microcontroller unit through the communication protocol. Consequently, at least one original firmware information in the at least one microcontroller unit is replaced by the at least one update firmware information.